My mechanic is basically that there's a real world and a reflected world, and stuff can show up in both or just one of these worlds.
Simple Demo
Do you have any idea for a puzzle that I could make with it? I guess ideas come with time, but I some feedback could help :D
Ideas to change/extend the mechanic in some levels are obviously very welcome

I've seen platformer games before utilising this kind of mechanic, and it usually works really well in them -- you have to account for obstacles in both the "proper" level and the reflected one.

Neat demo!

If you add in a 'mirror' that sends items from one side to the other, and give every item an attribute (real or shadow) and only allow things of that type to interact with each other, you could do something like:

• Shadow box is in color world
• Color person has to drop shadow box into mirror
• Shadow box appears in shadow world, and can now be used on the shadow-side switch

And instead of perfectly mirroring things, you could have, say, a shadow-only barrier that needs to be opened using a color-side switch before the shadow person can progress. (Looking at the GIF again, I see you do have a shadow-only box).

Or make it so only certain things are mirrored, and if it is something that appears in both worlds, then moving it in the color world will affect that thing's position in the shadow world.

Instead of having all movement tied together, I'd propose making it a co-op game. And a 1-player option where you would swap characters by pressing the X button.

I did implement enargy's proposal of a mirror, second level also looks different now after implementing two-way entities:
GIF
edit: fixed gif
edit made it more visible

Cool, I like it.

>Instead of having all movement tied together, I'd propose making it a co-op game. And a 1-player option where you would swap characters by pressing the X button.
I think having a fixed mirrored character is what should define the riddles.

Today I added some one way portals